Instruction manual

SM425 INSTRUCTION MANUAL 11.0 INSPECTION and MAINTENANCE
11.1.2 Replacing the Sensor Assembly
The performance of the sensor (cell) deteriorates as its surface becomes contaminated during operation.
The sensor must be replaced when it has reached its life expectancy. For example, when it can no
longer detect a zero-gas ratio of 100 ±30 % or a span-gas ratio of 0 ±18 %. The sensor assembly must be
replaced if it is damaged or becomes inoperable.
Determine the reason for the sensor malfunction and if possible initiate prevent action.
- To prevent burns, allow the detector to cool before replacing the sensor. Replace the metal O-ring and
contact when replacing the sensor. A deformed contact should always be replaced.
- Any corrosion or discoloring on the contact seating surface should be removed using sandpaper or a
metal brush. A final polishing should be made with a high grade of sandpaper (e.g. 1500 grit) or an
appropriate metal brush to eliminate any sharp protrusions on the surface. The contact’s electrical
resistance should be minimized.
- Use sensor assemblies manufactured on or after Sept. 2000. The serial number on the side of the
sensor assembly should be 0J000 or later (for example: OK123, 1AA01 etc).
11.1.2.1
11.1.2.2
11.1.2.3
Identifying Sensor Cell Parts to be Replaced
Identify the sensor parts that need to be replaced which typically are the sensor cell, metal O-ring, and
contact. Replace the U-shaped pipe, bolts, filter and associated spring washers if necessary.
Sensor Cell Removal Procedure
1. Remove the four bolts and associated washers from the tip of the detector probe.
2. Remove the U-shaped pipe support with the U-shaped pipe and the filter.
3. Pull the sensor assembly toward you while turning it clockwise. Remove the metal O-ring
between the assembly and the probe. Be careful not to scratch or dent the metal O-ring contact
surface on the tip of the probe. Use tweezers to pull the contact out of the groove in the tip of the
probe.
4. Clean the sensor assembly and the metal O-ring contact surfaces on both the sensor and probe.
Clean any parts that can be reused. The metal O-ring must be replaced to ensure a good seal.
Sensor Cell Assembly procedure
1. Install the contact being careful not to deform it. Position it in the ringed groove so that it forms a
solid contact with the probe.
176 42500001 REV A